The present invention relates to polymer blends and polymer blend membranes which consist of a polymeric sulfonic acid and of a polymer which contains primary, secondary or tertiary amino groups, which are prepared via mixing of a salt of the polymeric sulfonic acid with the polymer which contains primary, secondary or tertiary amino groups. The invention further relates to the use of polymer blend membranes in membrane fuel cells, polymer electrolyte membrane fuel cells (PEM fuel cells) or direct methanol fuel cells (DMFC), in membrane electrolysis, in aqueous or non-aqueous electrodialysis, in diffusion dialysis, in the perstractive separation of alkenes from alkene/alkane mixtures (here the membranes are in the SO3Ag form, where the Ag+ forms a reversible complex with the alkene (xe2x86x92facilitated transport)), in pervaporative separation of water from water/organics mixtures, or in gas separation.
A key cost component of the PEM fuel cell is its proton-conducting membrane. The perfluorinated ionomer Nafion(copyright) (Grot, W. G.: Perfluorinated Ion-Exchange Polymers and Their Use in Research and Industry, Macromolecular Symposia, 82, 161-172 (1994)) which has been commercially available meets the requirements of chemical stability which must be demanded of membranes for the application in PEM fuel cells (Ledjeff, K.; Heinzel, A.; Mahlendorf, F.; Peinecke, V.: Die reversible Membran-Brennstoffzelle, Dechema-Monographien Band 128, VCH Verlagsgesellschaft, 103-118 (1993)). However, it has various disadvantages which necessitate the search for alternative materials: It is very expensive (DM 1400.xe2x88x92/m2). The very complex production process comprises highly toxic intermediates (see Grot, W. G.). The environment-compatibility of Nafion(copyright) is to be evaluated critically: as a perfluorinated polymer, it is hardly degradable. The recyclability of Nafion(copyright) is questionable.
When applying Nafion(copyright) in direct methanol fuel cells, it was discovered that it shows a very high methanol-permeability, especially when pure methanol is used (Surampudi, S., Narayanan, S. R.; Vamos, E.; Frank, H.; Halpert, G.; LaConti, A.; Kosek, J.; Surya Prakash, G. K.; Olah, G. A.: Advances in direct oxidation methanol fuel cells, J. Power Sources, 47, 377-385 (1994)), which greatly reduces the energy efficiency of the DMFC by mixed potential formation.
Partially fluorinated ionomers are presently under investigation. At this point, the scientific work of G. G. Scherer may be mentioned (Scherer, G. G.: Polymer Membranes for Fuel Cells, Ber. Bunsenges. Phys. Chem. 94, 1008-1014 (1990)); (Scherer, G. G.; Bxc3xcchi, F. N.; Gupta, B.; Rouilly, M.; Hauser, P. C.; Chapiro, A.: Radiation Grafted and Sulfonated (FEP-g-Polystyrene)xe2x80x94An Alternative to Perfluorinated Membranes for PEM Fuel Cells? Proceedings of the 27th Intersociety Energy Conversion Engineering Conference IECEC-92, San Diego, USA, August 3-7, 3.419-3.424 (1992)); (Gupta, B.; Bxc3xcchi, F. N.; Scherer, G. G.: Materials Research Aspects of Organic Solid Proton Conductors Solid State Ionics 61, 213-218 (1993)), who formed free radicals in perfluorinated polymer foils using gamma radiation and grafted styrene onto the free radicals formed. Then, the polystyrene chains of the perfluoropolymer-polystyrene IPNs (interpenetrating polymer networks) formed were sulfonated. These polymer membranes showed a good performance when used in PEM fuel cells. However, the synthetic method employed seems to be unsuitable for mass production of this type of membrane. The Canadian company Ballard has developed a partially fluorinated proton-conducting membrane from sulfonated poly(xcex1,xcex2,xcex2-trifluorostyrene) (Wei, J.; Stone, C.; Steck, A. E.: Trifluorostyrene and substituted trifluorostyrene copolymeric compositions and ion-exchange membranes formed therefrom, WO 95/08581, Ballard Power Systems). A disadvantage of this membrane is its high price because of the complex production process for the monomer xcex1,xcex2,xcex2-trifluorostyrene (Livingston, D. I.; Kamath, P. M.; Corley, R. S.: Poly-xcex1,xcex2,xcex2-trifluorostyrene, Journal of Polymer Science, 20, 485-490 (1956)) and because of the poor capability of poly(xcex1,xcex2,xcex2-trifluorostyrene) of being sulfonated.
In the literature, some references are found relating to the application of arylene main-chain polymers to PEM fuel cells. The most important articles will be mentioned in the following:
Polybenzimidazole-phosphoric Acid
Membranes of the engineering thermoplastic polybenzimidazole are soaked with phosphoric acid (Wainright, J. S.; Wang, J.-T.; Savinell, R. F.; Litt, M.; Moaddel, H.; Rogers, C.: Acid Doped Polybenzimidazoles, A New Polymer Electrolyte, The Electrochemical Society, Spring Meeting, San Francisco, May 22-27, Extended Abstracts, Vol. 94-1, 982-983 (1994))xe2x80x94the phosphoric acid works as a proton conductor. The phosphoric acid molecules are held in the membrane by hydrogen bridges and through protonation of the imidazole moieties with formation of the salt H2PO4xe2x88x92+ HNpolymer. However, there is a risk with these membranes that the phosphoric acid is gradually washed out of the polybenzimidazole matrix with the water formed in the fuel cell during operation thereof, because the ratio of phosphoric acid molecules to imidazole moieties is about 3:1 in these polymer blends.
Sulfonated Polyethersulfone
An article by Ledjeff (Nolte, R.; Ledjeff, K.; Bauer, M.; Mxc3xclhaupt, R.: Partially Sulfonated poly(arylene ether sulfone)xe2x80x94A Versatile Proton Conducting Membrane Material for Modern Energy Conversion Technologies, Journal of Membrane Science 83, 211-220 (1993)) suggests the use of cross-linked sulfonated polyethersulfone ionomers, prepared by electrophilic sulfonation of polyethersulfone, as proton conductors in PEM fuel cells. However, no current-voltage characteristic of the presented membrane is given in this paper, which makes the evaluation of the suitability of this ionomer for PEM fuel cells difficult.
Sulfonated PEEK
In the patent literature, a reference dealing with the use of membranes of sulfonated polyetherketones (PEEK) in PEM fuel cells can be found (Helmer-Metzmann, F.; Ledjeff, K.; Nolte, R., et al.: Polymerelektrolyt-Membran und Verfahren zu ihrer Herstellung, EP 0 574 791 A2). These polymers are said to exhibit a good performance and chemical stability in PEM fuel cells. However, these membranes show high swelling values, especially at the high proton conductivities and thus ion-exchange capacities as required for PEM fuel cells, which deteriorates their mechanical properties and thus shortens their service life in fuel cells. In addition, especially when PEEK is sulfonated heterogeneously, there is a risk that the polymer partially recrystallizes (unmodified PEEK is partially crystalline), leading to brittleness.
Sulfonated Polyphenylenes
Membranes prepared from organic solvent soluble sulfonated, chemically and thermally stable polyphenylenes as alternative materials to replace Nafion(copyright) for use in PEM fuel cells are suggested by Matejcek, L.; Nolte, R.; Heinzel, A.; Ledjeff, K.; Zerfass, T.; Mxc3xclhaupt, R.; Frey, H.: Die Membranbrennstoffzelle: Untersuchungen an Membran/Elektrodeneinheiten, Jahrestagung 1995 der Fachgruppe Angewandte Elektrochemie der GDCh, Duisburg, 27.-29. Sept. 1995, Abstract Poster Nr. 20 (1995). However, no investigations of these membranes in PEM fuel cells have been published so far.
Sulfonated Polyphenylene Sulfide
Miyatake, K.; Iyotani, H.; Yamamoto, K.; Tsuchida, E.: Synthesis of Poly(phenylene sulfide sulfonic acid) via Poly(sulfonium cation) as a Thermostable Proton-Conducting Polymer, Macromolecules 1996, 29, 6969-6971 (1996), reports the preparation of a chemically and thermally stable sulfonated polyphenylene sulfide via a polysulfonium cation intermediate. A disadvantage of this preparation process is its being relatively complicated and thus expensive.
Acid-base polymer blends based on vinyl polymers are often mentioned in the relevant literature (Bazuin, C. G.: Ionomers (Compatibilization of Blends), in: Polymeric Materials Encyclopedia (Ed.-in-Chief J. C. Salomone), Vol. 5 (H-L), CRC Press (Boca Raton, New York, London, Tokyo) 3454-3460 (1996)), for example, those acid-base blends containing polymethacrylates as the acidic component and polyvinyl-pyridinium salts as the basic component (Zhang, X.; Eisenberg, A.: NMR and Dynamic Mechanical Studies of Miscibility Enhancement via Ionic Interactions in Polystyrene/poly(ethyl Acrylate) Blends, J. Polym. Sci.: Part B: Polymer Physics, 28, 1841-1857 (1990)). These acid-base blends have been investigated, e.g., in terms of compatibility between the acidic and basic blend components. Practical applications of these acid-base polymer blends have not become public so far.
As mentioned above, the provision of chemically stable ionomer membranes for electro-membrane processes, especially for membrane fuel cells, is an important area of research. The ionomer membranes should be selected from the group of arylene main-chain polymer membranes, because these polymers exhibit the highest chemical stability next to the perfluorinated polymers. Acid-base blends based on PEEK are described in Kerres, J.; Cui, W.; Wagner, N.; Schnurnberger, W.; Eigenberger, G.: A.7 Entwicklung von Membranen fxc3xcr die Elektrolyse und fxc3xcr Membranbrennstoffzellen, Vortrag, xe2x80x9cJahreskolloquium 1997 des Sonderforschungsbereichs 270xe2x80x94Energietrxc3xa4ger Wasserstoffxe2x80x9d, Sep. 29, 1997, Berichtsband p. 169-193 (1997); ISBN: 3-00-001796-8; Cui, W.; Kerres, J.; Eigenberger, G.: Development and Characterization of Ion-Exchange Polymer Blend Membranes, Poster, Euromembrane ""97, xe2x80x9cProgress in Membrane Science and Technologyxe2x80x9d, University of Twente, Jun. 23-27, 1997, Abstracts p. 181 (1997). The polymer blends are prepared by mixing poly(etheretherketonesulfonic acid) (PEEK SO3H) and poly(ethersulfone-ortho-sulfonediamine) (PSU-NH2) in a dipolar-aprotic solvent followed by evaporation of the solvent. The publications describe the characterization of these special polymer blends in terms of ionic conduction, swelling, permselectivity and thermal resistance, and the use of one of these membranes alone in electrodialysis. With the method described (mixing of the polymeric sulfonic acid with the polymeric amine), only those acid-base blends can be prepared which have a very weakly basic amine component, such as poly(ethersulfone-ortho-sulfone diamine). Stronger polymeric bases immediately form an insoluble polyelectrolyte complex upon mixing with the polymeric sulfonic acid.
On principle, all sulfonated aryl polymers exhibit a high brittleness when drying out, for example, when they are applied in fuel cells at intermittent conditions. The reduction in brittleness of the sulfonated aryl polymer ionomers has thus priority in their further development for long-term application in PEM fuel cells.
In a first embodiment, the above object is achieved by a process for the preparation of ion-exchange membranes, characterized in that solutions of polymeric sulfonic acid salts having the general formula
xe2x80x83polymer-SO3X,
where X=monovalent metal cations, NH4+, NH3R+, NH2R2+, NHR3+, NR4+, pyridinium, R=any alkyl and/or aryl radical, are reacted with polymers containing primary, secondary or tertiary nitrogen in dipolar-aprotic solvents, and the obtained polymeric sulfonic acid salt/base blends are aftertreated in hot diluted mineral acid at 20 to 100xc2x0 C. after storage.